WASHINGTON — The threat posed by the militant Islamic State group is "beyond anything we've seen," Defense Secretary Chuck Hagel said Thursday, because the group is "sophisticated and well funded."

The United States "will not relent our efforts to bring our citizens home and their captors to justice," Hagel said about the failed mission aimed at rescuing American hostages held by the group last month.

Journalist James Foley, who was executed by the group Tuesday, was included in that group of hostages, Hagel said.

Hagel and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told reporters at the Pentagon that the rescue mission was detailed publicly because media outlets were prepared to report about it.

"This operation, by the way, was a flawless operation, but the hostages were not there," Hagel said.

The Islamic State, Hagel said, is an imminent threat to the United States. Tuesday's beheading of Foley, which was posted online, shows that.

The effort to stop the Islamic State, Dempsey said, has involved seven humanitarian airdrop missions, more than 60 daily intelligence, surveillance and reconnaissance missions and 89 targeted airstrikes on Islamic State positions in northern Iraq. Air Force and Navy aircraft have participated in the airstrikes, Dempsey said.

Hagel said more nations would join the United States, Great Britain, France and other allies in the joint effort to stop the Islamic State.

"It is possible to contain" the Islamic State, Dempsey said, adding that the group's momentum has been stopped. He said the sense that the Islamic State was going to overrun much of Iraq led many people to acquiesce to its domination.